How to Make It
Place bones in a 12-quart stockpot and add cold water to cover by 2 inches. Bring to a boil over high heat. Remove from heat; pour off and discard liquid. Rinse bones under cold running water. Wash pot.
Meanwhile, toast cinnamon stick, cloves, allspice, coriander, and star anise in a dry skillet over moderately high heat, shaking pan occasionally, until spices are toasted and aromatic, about 1 minute. Place spices in cheesecloth; secure with butcher’s twine.
Return bones to pot and add 2 gallons cold water. Bring to a boil over high heat, skimming impurities as they rise to the surface, 25 to 30 minutes. Reduce heat to maintain a very gentle simmer. Add onions, ginger, and rock candy. Simmer 3 hours. Discard onions and ginger. Add spice pouch, flank steak, and 2 1/2 tablespoons salt; simmer until spices are infused, about 1 hour. Remove and discard pouch. Simmer gently until steak is cooked through and tender but not falling apart, about 1 hour.
Remove steak; let rest 15 minutes. Pour broth through a cheesecloth-lined fine wire-mesh strainer. Stir in remaining salt. Thinly slice steak against the grain.